Optical switching apparatus and optical communication network system

ABSTRACT

An optical communication network system is disclosed, which includes an optical switching apparatus. The optical switching apparatus includes an optical switch for switching and setting routes of an optical signal without being converted, a control unit for instructing the optical switch to execute a route switching operation, and a performance monitor for detecting performance of the optical signal having a route set by the optical switch. The performance monitor issues an alarm when the performance detected is deteriorated from predetermined performance. The control unit includes an alarm masking unit for masking the alarm issued from the performance monitor at least for a predetermined masking period from a starting time of a switching operation by the optical switch, and thus preventing the alarm from being issued.

BACKGROUND OF THE INVENTION

The present invention relates to an optical communication networksystem, and more particularly to an optical communication network systemfor performing switching of routes of an optical signal withoutconverting the optical signal into an electric signal.

In order to deal with a rapid increase in data traffic represented bythe Internet and a sudden increase in demand for multimediacommunications including images, audio and data, a higher speedoperation and a larger capacity operation have been pushed ahead for atransmission line and a communication node, which constitute acommunication network, and there have been progresses made in theintroduction of an optical communication apparatus using an opticalfiber and an optical signal. In addition, in place of a conventionalcommunication apparatus for processing an optical signal, in which theoptical signal is converted into an electric signal once, studies havebeen conducted on practical use of an optical cross-connect apparatus(OXC) and an optical add-drop multiplexing apparatus (OADM) forperforming switching process such as switching of transmissionroutes/signal lines without converting an optical signal into anelectric signal. The OXC and the OADM uses an optical switch as a maincomponent for switching optical transmission lines. As an opticalswitch, various types have been known, e.g., a mechanical opticalswitch, an optical switch using a thermooptical effect, an opticalswitch using an electrooptical effect and the like. Among these types,the mechanical optical switch is most often used because a power lossthereof is the smallest.

For the practical use of the OXC or the OADM, it is essential to providean apparatus, which is configured to improve basic performance such assuppression of a power loss of an optical signal or the like and to becapable of properly switching and operating signal routes (or a systemitself), and to be excellent in reliability, availability andserviceability (hereinafter referred to as “RAS function”). In aconventional transmitter or a digital switching device such as amultiplexer processing an electric signal, performance of a signal to beprocessed has been monitored in a proper position, or a redundantconfiguration (e.g., duplication) in a part of an apparatus has beenadopted. Thus, an apparatus having an excellent RAS function has beenprovided.

In the conventional case of using the electric signal, time multiplexingcan be carried out up to 10 Gbps and, in principle, routes can beswitched by using this technology. However, in a digital transmissionsystem such as SONET/SDH, to execute process of high-level controlmanagement signal, 64 signals of 155 Mbps are arrayed in paralleldevelopment, and routes are switched. For such a speed, a technology forswitching by using a data buffering technology without any momentarypower failures power-interruption has been known.

As described above, the OXC and the OADM uses the optical switch as themain component for switching routes of an optical signal. However, inthe OXC and the OADM directly processing the optical signal, thereoccurs a problem, in the case of the most often used mechanical opticalswitch, that a switching speed is slow, which is several milli-secondsat the shortest, while a transmission rate of the optical signal to bepassed is 10 Gbps or higher (e.g., 40 Gbps), which is much higher thanthat of an electric signal. Consequently, if switching of signal routessimilar to the conventional apparatus for processing an electric signalis simply executed for the OXC or the OADM, a momentary power failureoccurs, where an optical signal of several million bits, that is,several tens of frames, is lost because of its inability to pass throughthe optical switch during optical signal route switching by the opticalswitch. In other words, the momentary power failure that has beenprevented by the conventional apparatus for processing the electricsignal occurs in the OXC or the OADM directly processing the opticalsignal. Thus, a need arises to realize an optical signal switchingapparatus having an excellent RAS function on the assumption of presenceof a momentary power failure by an optical switch.

Generally, in the OXC or the OADM, in order to maintain performance ofan optical signal to be processed, after switching of optical routes,various factors are monitored, which include (1) optical signal powerdeterioration/failure [detection level: −20 dBm, detection time: orderof 1 μsec.], (2) synchronous state of an operation clock [detectiontime: order of 1 μsec.], (3) synchronous state of an optical signalframe [detection time: 375 μsec,], (4) optical signal error rate (biterror rate, referred to as BER, hereinafter) [detection level: 10⁻⁹,detection time: 10 sec.], and the like. This monitoring is carried outfor a predetermined time, and optical signal route (or system itself) isproperly switched to another when a trouble or a possibility of atrouble is discovered. Such a trouble monitoring function is essentialfor an improvement of the RAS function. The detection levels and thedetection times, which are bracketed in the above-described factors, areonly examples, and can be properly changed depending on a speed of anoptical signal to be processed by the apparatus or a size or installingplace of the apparatus.

In the apparatus provided with the above-described trouble monitoringfunction, depending on an installing position of a monitoring circuit ora monitoring method, a momentary power failure due to route switching bythe optical switch may be detected as an optical signal power failure,BER degradation or stepping-out of synchronization. Consequently, evenif the switching is a normal operation, a situation may occur where analarm is given to the downstream side of an optical signal advancingdirection or an apparatus for monitoring and controlling troubles. Inaddition, generally, the monitoring circuit also verifies a normal stateafter completion of the route switching or monitors recovery from thetrouble. Thus, unless monitoring is carried out by considering timenecessary for route switching by the optical switch or an operation timeof the above-described trouble monitoring function, even if theswitching has been normally carried out, a situation may occur where analarm is given to the downstream side of the optical signal advancingdirection or the apparatus for monitoring and controlling troubles. Inthe OXC or the OADM, such a situation induces repeating route switchingeven if an operation is normal. Consequently, an operation of the entireOXC or OADM, or an operation of a communication system (network) usingthe OXC or the OADM becomes unstable, it brings about a state when theRAS function can not be operated as desired. Needless to say, such asituation can be prevented by introducing a protective function forextending trouble detection time, recovery monitoring time and the like.However, such a method is not preferable for an improvement of the RASfunction because an original alarm monitoring ability is reduced.

Meanwhile, in the conventional communication apparatus for processing anelectric signal, such as a digital switching device and the like, theone has been known, which is configured to previously mask erroneousinformation caused by an in-apparatus operation (e.g., system switching,hardware maintenance/switching) based on software instruction in orderto prevent collection thereof and then to carry out an operation, tocollect by the software an alarm or management information monitored byhardware in the apparatus, and the like. However, the masking functionby the software in the conventional communication apparatus forprocessing an electric signal cannot simply be applied to the OXC or theOADM.

As a specific example, when an optical route is switched by the opticalswitch, a momentary power failure causes an optical signal powerfailure, and stepping-out of clock synchronization (hereinafter referredto as clock stepping-out) and stepping-out of frame synchronization(hereinafter referred to as frame stepping-out) of a transmissionsignal. However, recovery from the optical signal power failure isdetected during switching time (about 1 milli-sec.) after completion ofswitching. Meanwhile, for the clock stepping-out and the framestepping-out, after optical signal power is recovered by a newconnection, new clock and frame synchronization must be performed. Timenecessary for verifying re-synchronization exceeds 1 milli-sec. Further,10000 frames are necessary for BER measurement since framesynchronization is secured, and the process must wait for 10 sec.Consequently, when correct operation of route switching is carried outby distinguishing a momentary power failure due to switching of theoptical switch from a disconnection of an optical fiber as a fixedtrouble, if only the conventional trouble detection method or theconventional masking function by the software simply is applied to theOXC or the OADM, the RAS function becomes short. Thus, there is a demandfor an OXC or an OADM having an excellent RAS function for detecting areal trouble and switching routes in consideration of a combination of aplurality of factors for trouble detection and monitoring time thereofwith a trouble detection/recovery detection operation carried outfollowing disposition of a trouble detection circuit in an apparatus.Furthermore, there is a demand for an OXC or an OADM preventingnotification of an alarm to a downstream side of an optical signaladvancing direction or an apparatus for monitoring and controlling atrouble even if a momentary power failure occurs due to route switching,and preventing induced re-switching of routes while an operation isnormal.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a highly reliableoptical communication network system for switching routes without anyconversion of an optical signal, which is preventing notification of anerroneous alarm during switching by an optical switch.

In order to achieve the above-described object, according to the presentinvention, a configuration is adopted, where an alarm-issued from amonitor for monitoring performance of an optical signal having a routeset by an optical switch is masked for a predetermined masking periodfrom a starting time of a switching operation by the optical switch.Thus, even if an alarm is issued because of a change in performance ofan optical signal by a normal switching operation of the optical switch,it is possible to prevent the alarm from being recognized as such by thesystem.

The monitor can be adapted to detect the performance of the opticalsignal regarding a plurality of predetermined factors, and to issue analarm for each of the plurality of factors. In this case, theabove-described masking period should preferably be set for each of theplurality of factors. Accordingly, it is possible to mask an alarm foreach of the plurality of factors and each of the detected factors ofoptical signal performance only for a minimum necessary period, whileoptical signal performance is reduced because of the normal switchingoperation of the optical switch.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which:

FIG. 1 is an explanatory view showing an entire configuration of anoptical communication network system according to an embodiment of thepresent invention;

FIG. 2 is a block diagram showing a basic configuration of an opticalroute setting apparatus 100 used for the optical communication networksystem of the embodiment of the present invention;

FIG. 3 is a flowchart showing a process for switching of optical routesin the optical route setting apparatus 100 of FIG. 2;

FIGS. 4(a) to (h) are explanatory views showing states of respectiveportions and signal time charts when an optical route switchingoperation is normally carried out in the optical route setting apparatus100 of FIG. 2;

FIGS. 5(a) to (h) are explanatory views showing states of the respectiveportions and signal time charts when an optical power failure alarm isissued after an optical route switching operation in the optical routesetting apparatus 100 of FIG. 2;

FIGS. 6(a) to (h) are explanatory views showing states of the respectiveportions and signal time charts when an error rate alarm is issued afterthe optical route switching operation in the optical route settingapparatus 100 of FIG. 2;

FIG. 7 is a block diagram showing a specific configuration of theoptical route setting apparatus 100 used for the optical communicationnetwork system of the embodiment of the present invention;

FIG. 8 is an explanatory view showing a state of each bit area of analarm register 352 and a mask register 363 in a state where a switchingoperation of the optical route setting apparatus 100 of FIG. 7 is notcarried out, and no alarms are issued;

FIG. 9 is an explanatory view showing a masking period and an outputafter masking for each alarm, which are stored in an alarm managementmemory 344 of the optical route setting apparatus 100 of FIG. 7;

FIG. 10 is an explanatory view showing states 1 to 5 of bit areas offailure alarms and error rate alarms of the alarm register 352 and themask register 353 of the optical route setting apparatus 100 of FIG. 7;

FIG. 11 is an explanatory view showing an optical fiber 2006 partiallydisposed double to solve a trouble in a ring of the optical fiber 2006of the optical communication network system of FIG. 1;

FIG. 12 is a block diagram showing a configuration of an opticaladd-drop multiplexing apparatus (OADM) of the optical communicationnetwork system of FIG. 1;

FIG. 13 is a block diagram showing a configuration of an opticalcross-connect apparatus (OXC) of the optical communication networksystem of FIG. 1;

FIG. 14 is a block diagram showing a configuration of the optical routesetting apparatus 100 of the embodiment of the present invention, wherean alarm mask is achieved by software;

FIG. 15 is a flowchart showing an operation of a CPU 342 of the opticalroute setting apparatus 100 of FIG. 14;

FIG. 16 is a flowchart showing an operation of the CPU 342 of theoptical route setting apparatus 100 of FIG. 14; and

FIG. 17 is a flowchart showing an operation of the CPU 342 of theoptical route setting apparatus 100 of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, description will be made for an optical communication networksystem according to an embodiment of the present invention.

As shown in FIG. 1, an optical communication network system of thisembodiment includes optical add-drop multiplexing apparatuses (OADM)1003 to 1009 and optical cross-connect apparatuses (OXC) 1001 and 1002,which are connected through optical fibers 2001 to 2006. Specifically,the optical add-drop multiplexing apparatuses (OADM) 1003 to 1005 areconnected in a ring shape through the optical fiber 2005, and opticaladd-drop multiplexing apparatuses (OADM) 1006 to 1009 are connected in aring shape through the optical fiber 2006. The optical fiber 2005 andthe optical fibers 2001 to 2004 are connected by the opticalcross-connect apparatus (OXC) 1001. The optical cross-connect apparatus(OXC) 1001 is also connected to the optical add-drop multiplexingapparatus (OADM) 1006. The optical fiber 2003 also are connected toother optical fibers through the optical cross-connect apparatus (OXC)1002.

As shown in FIG. 12, the optical add-drop multiplexing apparatus (OADM)1003 includes a divider 121 for division (demultiplexing) an opticalsignal having been subjected to wavelength-multiplexing, an opticalroute setting apparatus (an optical switching apparatus) 100 forswitching routes of an optical signal, and a multiplexer 122 formultiplexing optical signals. Thus, a multiplexed optical signalreceived through the optical fiber 2005 is divided by the wavelengthdivider 121, and a necessary optical signal is taken out by the opticalroute setting apparatus 100 and output through the optical fiber 2007 toan external apparatus. The optical signal received from the externalapparatus through the optical fiber 2007 is multiplexed with the otheroptical signal by a wavelength multiplexer 122 and sent to the opticalfiber 2005. The optical add-drop multiplexing apparatuses (OADM) 1004 to1009 are similar in configuration to the optical add-drop multiplexingapparatus (OADM) 1003. That is, a necessary optical signal is taken outfrom multiplexed optical signals received through the optical fibers2005 and 2006, and output through optical fibers 2008 to 2013 to theexternal apparatus. The optical signal received from the externalapparatus is multiplexed with the other optical signal, and then sent tothe optical fibers 2005 and 2006.

Meanwhile, as shown in FIG. 13, the optical cross-connect apparatus(OXC) 1001 includes divider 131, 132 and the like for division(demultiplexing) an optical signal having been subjected towavelength-multiplexing, an optical route setting apparatus 100 forswitching routes of an optical signal, and multiplexers 133, 134 and thelike for multiplexing an optical signal. Thus, a multiplexed opticalsignal received through the optical fibers 2001, 2005 and the like isdivided by the divider 131, 132 and the like, and a route is switched bythe optical route setting apparatus 100 to an optical fiber, whichbecomes a destination for each optical signal. Then, the optical signalis multiplexed again by the multiplexers 133, 134 and the like, andoutput to the optical fibers 2005, 2004 and the like. In addition, theoptical cross-connect apparatus (OXC) 1001 transmits and receivers anoptical signal locally with the optical add-drop multiplexing apparatus(OADM) 1006 through the optical fiber 2010. The optical cross-connectapparatus (OXC) 1002 is also similar in configuration to the opticalcross-connect apparatus (OXC) 1001.

As described above, each of the optical add-drop multiplexingapparatuses (OADM) 1003 to 1009 and the optical cross-connectapparatuses (OXC) 1001 and 1002 include the optical route settingapparatus 100 for switching routes without converting an optical signalinto an electric signal. The optical route setting apparatus 100includes an optical switch 300 for switching routes of an opticalsignal, a control unit 305 for controlling an operation of the opticalswitch 300, and optical performance monitors 310. The opticalperformance monitor 310 detects four factors: power deterioration of anoptical signal passed through the optical switch 300; a synchronousstate of an operation clock; a synchronous state of an optical signalframe; and an error rate of an optical signal. A result of the detectionby the optical performance monitor 310 is supplied to the control unit305.

The optical communication network system of the embodiment has astructure for recovery from a trouble in the case of occurrence thereof.Specifically, a transmission line is constructed by connecting theoptical add-drop multiplexing apparatuses (OADM) 1006 to 1009 in a ringshape through the optical fiber 2006. As shown in FIG. 11, in thistransmission line the optical fibers 2006-4 connecting between theoptical add-drop multiplexing apparatuses (OADM) 1006 and 1007 areconstituted of two parallel optical fibers 2006-4 a and 2006-4 b, andthe optical add-drop multiplexing apparatuses (OADM) 1006 and 1007 canselect any one of the optical fibers 2006-4 a and 2006-4 b by aswitching operation of the optical route setting apparatus 100 totransmit an optical signal. In the transmission line of FIG. 11,normally, a communication route for transmitting an optical signal fromthe optical fiber 2010 to the optical fiber 2012 is preset to passthrough the optical add-drop multiplexing apparatus (OADM) 1006, theoptical fiber 2006-4 a, the optical add-drop multiplexing apparatus(OADM) 1007, the optical fiber 2006-3, the optical add-drop multiplexingapparatus (OADM) 1008, and the optical fiber 2012. It is also arrangedbeforehand that a trouble occurring in the optical fiber 2006-4 a issolved by switching to the optical fiber 2006-4 b to transmit theoptical signal. It is further arranged beforehand that troublesoccurring in both of the optical fibers 2006-4 a and 2006-4 b are solvedby switching the transmission line to pass through the optical add-dropmultiplexing apparatus (OADM) 1006, the optical fiber 2006-1, theoptical add-drop multiplexing apparatus (OADM) 1009, the optical fiber2006-2, the optical add-drop multiplexing apparatus (OADM) 1008, and theoptical fiber 2012. These switching operations for recovery fromtroubles are carried out under a supervisory and control system (OpS),not shown, which instruct route switching to the control unit 305 of theoptical route setting apparatus 100 of each of the optical add-dropmultiplexing apparatuses (OADM) 1006 to 1009, or under self-judgment ofthe control unit 305.

In the embodiment, a mechanical optical switch is used for the opticalswitch 300 of the optical route setting apparatus 100. This opticalswitch 300 includes optical fibers disposed with end surfaces facingeach other, and a driver for mechanically shifting one of the opticalfibers vertically. By moving the optical fiber with the driver, apositional relation is set, where the optical switch faces end surfacesof optical fibers disposed adjacently to each other, and carries outswitching. Such a mechanical optical switch requires several milli-sec.at the shortest from a start of a switching operation to an end thereofand, during this period, a momentary power failure occurs, where theoptical signal cannot be passed through the optical switch 300 and lost.In addition, during the switching operation, stepping-out of anoperation clock synchronization of a signal and stepping-out of anoptical signal frame synchronization thereof occur. Thus, even when aset route of the optical route setting apparatus 100 is changed withoutany troubles or the like, during the switching operation of the opticalswitch 300, a momentary power failure or stepping-out is detected by theoptical performance monitor 310. When a trouble is recognized while theswitching operation is actually normal, a recovery operation from thetrouble is started as described above with reference to FIG. 11. Thus,according to this embodiment, the optical route setting apparatus 100 isconstructed in the following manner, and an optical communicationnetwork system excellent in reliability, availability and serviceabilityis provided by using the optical switch 300.

By properly selecting components, the optical route setting apparatus100 of the embodiment can readily construct a flexible communicationnetwork capable of dealing with various transmission rates andmultiplexing degrees of optical signals. For example, an optical signalor the like having a transmission rate of STM-0 (51.84 MHz) set by ITU-TRecommendation can be used, and there are no limitations on presence ofwavelength-division-multiplexing or the number thereof.

First, description will be made for features of the optical routesetting apparatus of this embodiment with reference to a simplifiedconfiguration of FIG. 2. In the simplified optical route settingapparatus 100 of FIG. 2, attention is paid to a switching operation ofone of the optical switches N×N or N×M of FIG. 12 or 13, and two inputsignals 1 and 2, and one output signal are shown. The input signals 1and 2 are respectively inputted from routes 201 and 202 to the opticalswitch 300, and one selected by the optical switch 300 is outputted froma route 203. The optical signal outputted from the route 203 is passedthrough the optical performance monitor 310, and is detected about fourfactors, i.e., power deterioration of the optical signal, a synchronousstate of an operation clock, a synchronous state of an optical signalframe and an error rate of the optical signal. Then, if each factor islower than a predetermined value, an alarm is issued for each of thefour factors. The control unit 305 includes a system control unit 302,an alarm mask 303, and a timer 304. The system control unit 302 controlsroute switching of the optical switch 300, and collects alarminformation from the optical performance monitor 310 through the alarmmask 303. The alarm mask 303 includes an optical power failure alarmmask 303 a, an operation clock synchronization stepping-out alarm mask303 b (herein after referred to as “operation clock stepping-out alarmmask 303 b”), a frame synchronization stepping-out alarm mask 303 c(herein after referred to as “operation clock stepping-out alarm mask303 c”) and an error rate alarm mask 303 d corresponding to factors, forwhich the optical performance monitor 310 issues alarms during opticalroute switching.

Hereinafter, description will be made for an optical route switchingoperation of the optical route setting apparatus 100 by referring to theflowchart of FIG. 3. After an input of a route switching request fromthe supervisory and control system (OpS) or the like, not shown, to thesystem control unit 302 (step 400), the system control unit 302 actuatesa timer 304 by a trigger signal and sets the alarm mask 303 in a maskingstate (step 420). The system control unit 302 has a built-in memory, inwhich masking periods of the optical power failure alarm mask 303 a, theoperation clock stepping-out alarm mask 303 b, the frame stepping-outalarm mask 303 c and the error rate alarm mask 303 d are prestored. Thesystem control unit 302 reads each stored masking period, and sets themasking period in the timer 304 corresponding to an alarm of each of thefour factors. In this embodiment, the masking periods of the opticalpower failure alarm mask 303 a, the operation clock stepping-out alarmmask 303 b and the frame stepping-out alarm mask 303 c are set equal to10 ms, and the masking period of the error rate alarm mask 303 d is setequal to 15 s. In the case of a normal switching operation, thesemasking periods are preset to individual periods necessary until anormal state is recovered from the alarm state.

After the setting of the alarm mask 303, the system control unit 302sends a switching command signal to the driver of the optical switch300, and the driver switches optical routes (step 480). Immediatelyafter the switching of the optical routes, a failure occurs in theoptical signal, operation clock stepping-out and optical signal framestepping-out occur, and an error rate cannot be correctly measured.Therefore, alarms are issued from the optical performance monitor 310.However, since these have been masked respectively by the alarm masks303 a, 303 b, 303 c and 303 d, the system control unit 302 recognizes noalarms. After completion of the optical route switching, and passage ofthe set masking period, the timer 304 outputs mask releasing signalsrespectively to the above-described four alarm masks 303 a, 303 b, 303 cand 303 d (step 450), and releases the masks (step 460). When all thealarm masks 303 a, 303 b, 303 c and 303 d are released (step 470), theprocess returns to a normal alarm monitoring state for the four alarms(step 490).

Now, description will be made for an operation of the optical routesetting apparatus 100 by referring to time charts of FIGS. 4(a) to (h),FIGS. 5(a) to (h), and FIGS. 6(a) to (h).

First, by using the time charts of FIGS. 4(a) to (h), description ismade for a case where the optical switch 300 of the optical routesetting apparatus of FIG. 2 is normally switched.

Here, it is assumed that the optical switch 300 switches a signaloutputted from the output route 203 from an optical signal 1 to anoptical signal 2. A state changed with time is shown in FIG. 4(h). Astate before switching is a “STATE 1” 511 of FIG. 4(h). When there is aswitching request, the system control unit 302 executes the operation ofstep 420 to set masks in the optical power failure alarm mask 303 a, theoperation clock stepping-out alarm mask 303 b, the frame stepping-outalarm mask 303 c and the error rate alarm mask 303 d. Accordingly, a“STATE 2” 512 of FIG. 4(h) is set. Here, states of only an optical powerfailure alarm mask 504 and an error rate alarm mask 505 are shown inFIGS. 4(e) and (f).

The system control unit 302 executes the operation of step 430 to send aswitching command signal (FIG. 4(a)) to the driver of the optical switch300. In the optical switch 300 operates the driver operates to changemechanically a connection state, realizing a change from connection 1 toconnection 2, and a selected signal (FIG. 4(c)) of the optical switch300 is switched from the optical signal 1 to the optical signal 2. Timenecessary for this switching operation of the optical switch 300 isabout several ms, and a state executing this switching operation is a“STATE 3” 513 of FIG. 4(h).

In the “STATE 3” 513, the optical signal cannot be passed through theoptical switch 300. Thus, a power failure of the optical signal occurs,and an optical power failure alarm is issued from the alarm monitor 310.Moreover, since the error rate cannot be measured due to the opticalpower failure, the error rate alarm is also issued. In this “STATE 3”513, since masks are set in the optical power failure alarm mask 303 aand the error rate alarm mask 303 d as shown in FIGS. 4(e) and (f), noalarm signals are outputted from the alarm masks 303 a and 303 d to thesystem control unit 302 as shown in FIG. 4(g), and the system controlunit 302 recognizes a state as normal. After completion of the switchingoperation of the optical switch 300, the optical power failure alarm isreleased because of recovery of the optical power. Then, the opticalpower failure alarm mask 303 a is released by the operations of steps450 and 460, a “STATE 4” 514 is set. Though not shown, in the “STATE 3”513, operation clock stepping-out and frame stepping-out occur followingthe switching operation, and alarms are issued from the opticalperformance monitor 310. However, since there are masks set in theoperation clock stepping-out alarm mask 303 b and the frame stepping-outalarm mask 303 c, no alarm signals are outputted to the system controlunit 302. During this masking period, the optical performance monitor310 re-takes operation clock synchronization and frame synchronization,and releases the alarms. In addition, the optical performance monitor310 starts measurement of an error rate in matching with the opticalpower recovery. The error rate measurement takes time, an error ratealarm is continued, and a normal state is recovered after a passage ofcertain time. Then, the error rate alarm mask 505 has its mask releasedafter a passage of time required for measurement (steps 450 and 460),and a “STATE 5” 515 is set.

As a result, no alarms are issued from the alarm mask 303 to the systemcontrol unit 302 during switching to a normal signal, always setting anormal state.

Next, description will be made for a case where, after the switchingoperation of the optical switch 300 of the optical route settingapparatus of FIG. 2, an optical signal power failure state is set byreferring to time charts of FIGS. 5(a) to (h).

Causes of an optical signal power failure state after switching are, forexample, a case where an intensity itself of the optical signal 2 inputto the optical switch 300 is weak, a case where the switching operationof the optical switch 300 is not carried out normally, and the like. Inthis case, until a “STATE 2” 512 of FIG. 5(h) is similar to the “STATE2” 512 of FIGS. 4(a) to (h). In a “STATE 3” 513, an optical signal powerfailure occurs by the switching operation of the optical switch 300, andan optical power failure alarm and an error rate alarm are issued fromthe optical performance monitor 310. The optical power failure alarmmask 303 a is masked until a “STATE 3” 513 where the switching of theoptical switch 300 is completed. In a normal case similar to that shownin FIG. 4(d), the optical power failure alarm is released before an endof the “STATE 3” 513. However, if the optical signal 2 is abnormal andits intensity is weak as shown in FIG. 5(c), the optical power failurealarm of the optical performance monitor 310 continues even after themask of the optical power failure alarm mask 303 a is released. Thus, ata point of time when a “STATE 4” 514 is set, an optical power failurealarm is issued from the optical power alarm mask 303 a to the systemcontrol unit 302, and the system control unit 302 recognizes the failurealarm. In this case, since the optical power failure continues, an errorrate alarm also continues. From a point of time when a “STATE 5” 515 isset, an error rate alarm is issued from the error rate alarm mask 303 dto the system control unit 302, and the system control unit 302recognizes the error rate alarm. Accordingly, the system unit 302notifies an occurrence of a trouble to the supervisory and controlsystem (OpS) and, under instruction of the supervisory and controlsystem (OpS), a new route switching operation is started to makerecovery from the trouble shown in FIG. 11.

Next, description will be made for a case where an error rate of anoptical signal is reduced after the switching operation carried out bythe optical switch 300 of the optical route setting apparatus of FIG. 2,by referring to the time charts of FIGS. 6(a) to (h).

Causes of deteriorations in error rates after switching are, forexample, a case where there is much noise in an optical signal 2 itselfinputted to the optical switch 300, a case where interference occurs inthe optical switch 300 because of the switching operation of the opticalswitch 300. In this case, a state until a “STATE 4” 514 of FIG. 6(h) issimilar to a state until the “STATE 4” 514 of FIGS. 4(a) to (h). In thecase of normal switching, as shown in FIG. 4(d), until the period of the“STATE 4” 514, the error rate alarm is released. However, in the case ofFIG. 6(d), since the error rate is reduced in the optical signal 2, theerror rate alarm continues even when a “STATE 5” 515 is set, where theerror rate alarm mask 303 d is released in FIG. 6(f). Accordingly, inthe “STATE 5” 515, an error rate alarm is issued from the error ratealarm mask 303 d to the system control unit 302, and the system controlunit 302 then recognizes the error rate alarm. Thus, the system controlunit 302 notifies an occurrence of a trouble to the supervisory andcontrol system (OpS) and, under instruction of the supervisory controlsystem (OpS), a new route switching operation is started to makerecovery from the trouble, as shown in FIG. 11.

Next, description will be made for a specific configuration of theoptical route setting apparatus 100 of the embodiment by referring toFIG. 7. This optical route setting apparatus 100 monitors opticalperformance in an optical synchronous network (SONET) of 10 Gbit/s, anduses an optical switch 300 of switching time 1 ms. The optical routesetting apparatus 100 includes N×N optical switches 300, an opticalperformance monitor 310 disposed in each of output routes, amounting toN in number, of the optical switches 300, and a control unit 305.

The optical performance monitor 310 includes an optical splitter 361 forsplitting an output of the optical switch 300, a power monitor 362 fordetecting whether or not split optical power is equal to a predeterminedvalue or higher, a photoelectric converter 363 for converting an opticalsignal output from the optical switch 300 into an electric signal, aperformance monitoring circuit 364 for evaluating clock synchronization,frame synchronization and a bit error rate regarding the electric signalobtained by the conversion, and an electrooptical signal converter 365for re-converting the electric signal into an optical signal. The powermonitor 362 issues an optical power failure alarm when power of anoptical signal is lower than a predetermined value. The performancemonitoring circuit 364 monitors synchronous states of a reference clockoutput from a built-in reference clock circuit with clock and framesignals extracted from a received electric signal, outputs an operationclock stepping-out alarm and a frame stepping-out alarm whenstepping-out occurs, takes synchronization again within a fixed time,and stops the alarms when synchronization is established. In addition,the performance monitoring circuit 364 detects a bit error rate of anelectric signal, and issues an error rate alarm when the error rate islowered than a predetermined value.

The control unit 305 includes an alarm masking unit 303, a timer 346,and a portion equivalent to the system control unit 302 of FIG. 2. Inthe configuration of FIG. 7, the portion equivalent to the systemcontrol unit 302 includes a CPU 342, a switching information memory 343,an alarm management memory 344, a switching control circuit 345 and anI/O unit 341.

The alarm masking unit 303 includes an alarm interface circuit 354 forreceiving four kinds of alarms from the optical performance monitors 310amounting to N in number, an alarm register 352 for storing the alarms,a mask register 353 for setting an alarm mask, and an alarm issuing unit351. As shown in FIG. 8, corresponding to output ports (routes),amounting to N in number, of the optical switch 300, the alarm register352 has areas (bit areas) for writing “0” (normal) or “1” (there is analarm), which indicate whether or not the optical performance monitor310 issues a failure alarm, an error rate alarm, an operation clockstepping-out alarm and a frame stepping-out alarm. Similarly,corresponding to the output ports (routes), amounting to N in number, ofthe optical switch 300, the mask register 353 has areas (bit areas) forwriting “0” (mask is set)” or “1” (mask is released) indicating whetheror not masks are respectively set in a failure alarm, an error ratealarm, an operation clock stepping-out alarm and a frame stepping-outalarm. The alarm issuing unit 351 of the mask register 353 obtains aproduct of “0” or “1” written in the corresponding areas of the alarmregister 352 and the mask register 353, and outputs the alarm of itsoutput port, if “1”, to the CPU 342.

Writing of “0” or “1” in the alarm register 352 is carried out by thealarm interface circuit 354. Writing of “0” or “1” in the mask register353 is carried out by the CPU 342, which refers to a masking period ofeach alarm prestored in the alarm management memory 344 as shown in FIG.9, and operates the timer 346. In the switching information memory 343,a switching state of the optical switch 300 is stored. The maskingperiod of each alarm prestored in the alarm management memory 344 hasbeen decided based on the use of optical performance monitoring in theoptical synchronous network (SONET) of 10 Gbit/s and an optical switchof switching time 1 ms. In this case, as shown in FIG. 9, the maskingperiods are set at 10 ms for a failure alarm, an operation clockstepping-out alarm and a frame stepping-out alarm, and at 15 s for anerror rate alarm.

Now, description will be made concretely for an operation of the controlunit 305 of the optical route setting apparatus 100 of FIG. 7.

First, description will be made for a state where an input port 201-1 isconnected to an output port 203-N by the optical switch 300 and normallyoperated (no switching operations are carried out). In the mask register353, as shown in FIG. 8, “1” is written, indicating that all masks arereleased. Since there are no alarms issued from the optical performancemonitor 310, a value of the alarm register 352 is “0” indicating thatall are normal. This case is a “STATE 1” 611 shown in FIG. 10. Thus, noalarms are issued from the alarm issuing unit 351 to the CPU 342. FIG.10 shows only a failure alarm and an error rate alarm as kinds ofalarms, and an operation clock stepping-out alarm and a framestepping-out alarm are not shown. In this case, the timer 346 is reset.

When a bit error trouble occurs in an optical signal of the input port201-1, a degradation in a bit error rate is detected by the performancemonitoring circuit 364 in the optical performance monitor 360 of theoutput port 203-N, and an alarm is issued. This alarm is received by thealarm interface circuit 354, and “1” indicating presence of an alarm iswritten in a bit area corresponding to a bit error rate of a port N ofthe alarm register 352. In this case, since 1 indicating mask releasingis set in the mask register 358, an alarm is issued from the alarmissuing unit 351, and then received by the CPU 342. Determining that atrouble has occurred, the CPU 342 notifies the trouble to thesupervisory and control system (OpS) and, under instruction of thesupervisory and control system (OpS), a route switching operation isstarted for recovery from the trouble shown in FIG. 11.

Next, description will be made for an operation when the CPU 342receives, through the I/O unit 341, instruction to switch the input portconnected to the output port 203-N of the optical switch 300 from theinput port 201-1 to the input port 201-N.

Before issuing a switching command of the optical switch 300, the CPU342 sets the alarm mask 303 and starts the timer 346. First, “0”indicating mask setting is written in each of bit areas corresponding toan optical power failure alarm, an error rate alarm, an operation clockstepping-out alarm and a frame stepping-out alarm of the output port203-N of the mask register 353 of the alarm mask 303. Then, a maskingperiod of each alarm is read from the alarm management memory 344, setin the timer 346, and the timer 346 is started. Accordingly, a “STATE 2”612 of FIG. 10 is set. In response to a switching request received fromthe I/O unit 341, the CPU 342 calculates a new switching state from theswitching information memory 343, and outputs a switching command fromthe switching control unit 345 to the optical switch 300. The opticalswitch 300 operates the driver to connect the input port 201-N to theoutput port 203-N according to the switching command. The CPU 342 storesa new state in the switching information memory 343.

By the switching operation of the optical switch 300, an optical powerfailure occurs in an output optical signal of the output port 203-N. Thepower monitor 362 of the optical performance monitor 310 of the outputport 203-N detects the optical power failure, and issues an opticalpower failure alarm. The optical power failure alarm is received by thealarm interface circuit 354, and the alarm interface circuit 354 sets avalue of a bit area corresponding to the optical power failure alarm ofthe port N of the alarm register 352 to “1” indicating presence of analarm. The performance monitoring circuit 364 detects an error ratedegradation, operation clock stepping-out and frame stepping-out, andissues respective alarms. These alarms are received by the alarminterface circuit 354, and the bit areas of the error rate alarm, theoperation clock stepping-out alarm and the frame stepping-out alarm ofthe port N of the alarm register 352 are set to “1” indicating presenceof an alarm. In this case, as described above, in the mask register 353,“0” indicating masking is set in the bit area corresponding to eachalarm of the port N. Accordingly, a result of obtaining a logicalproduct of the alarm register 362 and the mask register 353 by the alarmissuing unit 351 is “0”, and thus the alarm issuing unit 351 issues noalarms. This is a “STATE 3” 613 of FIG. 10. In the case of an opticalpower failure, since an error rate cannot be measured, the performancemonitoring circuit 364 stops error rate measurement until optical poweris verified, and resumes the error rate measurement after recovery ofthe optical power.

Switching by the optical switch 300 is completed after about 1 ms fromthe reception of the switching command, and the optical signal reachesthe output port 203-N. Since the optical signal also reaches the powermonitor 362, the detection of the optical power failure alarm isreleased and, by the alarm interface circuit 354, “0” indicating anormal state is written in the bit area of the optical power failurealarm of the port N of the alarm register 352. After 10 ms from theswitching, optical power failure alarm mask releasing time is notifiedfrom the timer 353 to the CPU 342, and “1” indicating mask releasing iswritten in the optical power failure bit area of the mask register 353.In this case, since the optical power failure alarm has been released,the alarm issuing unit 351 gives no alarms to the CPU 342. This is a“STATE 4” 614 of FIG. 10.

The performance monitoring circuit 364 of the optical performancemonitor 310 of the output port 203-N resumes the error rate measurementafter recovery from the optical power failure and, with a passage of apredetermined time (about 10 sec.), an error rate of 10⁻⁹ or lower canbe measured. Accordingly, the error rate alarm is released and, throughthe alarm interface circuit 354, “0” indicating a normal state iswritten in the bit area of the error rate alarm of the port N of thealarm register 352.

After 15 sec., the error rate alarm releasing time is notified from thetimer 346 to the CPU 342, and “1” indicating mask releasing is writtenin the bit area of the error rate alarm of the port N of the maskregister 353. In this case, since the error rate alarm has beenreleased, and “0” has been written in the corresponding bit area of thealarm register, the alarm issuing unit 361 gives no alarms to the CPU342. After all the masks are released, the process returns to a normalstate “STATE 5” 615.

If the corresponding bit area of the alarm resister 352 is “1”indicating an abnormal state even when a mask releasing time specifiedby the timer 346 is reached, and “1” indicating mask releasing iswritten in the mask register 353, the alarm issuing unit 351 issues analarm to the CPU 342. Determining that a trouble has occurred, the CPU342 notifies the trouble to the supervisory and control system (OpS)and, under instruction of the supervisory and control system (OpS), anew route switching operation is started for recovery from the troubleshown in FIG. 11, or automatic switching is carried out forself-recovery.

As described above, the optical route setting apparatus 100 of theembodiment executes alarm masking for an optical signal power failure,an error rate degradation, operation clock stepping-out and framestepping-out, which occur following the normal switching operation ofthe optical switch 300, and thus erroneous recognition of an occurrenceof troubles can be prevented. That is, alarms received from theperformance monitor 310 are distinguished between (1) one caused by atrouble and (2) one caused by a route switching operation of the opticalswitch 300, and issuance of an alarm to the other connected apparatus iscontrolled. Specifically, in the case of (2), an alarm is prevented frombeing issued to the other optical add-drop multiplexing apparatus (OADM)1003, the optical cross-connect apparatus (OXC) 1001 or the supervisoryand control system (OpS) (not shown), which is connected through theoptical fibers 2005, 2006 or the like to the self apparatus. Thus, thereis no possibility of starting a new route switching operation or thelike for recovery from a trouble despite of the normal switchingoperation of the optical switch 300. Therefore, it is possible toprovide a highly reliable optical route setting apparatus.

In addition, in the optical route setting apparatus 100 of theembodiment, proper alarm masking periods can be set for the plurality offactors, i.e., an optical signal power failure, an error ratedegradation, operation clock stepping-out and frame stepping-out.Accordingly, without reducing detection accuracy of an occurrence ofreal troubles, erroneous recognition of an alarm following the normalswitching operation of the optical switch can be prevented, thusenhancing reliability.

Therefore, by using the optical add-drop multiplexing apparatus (OADM)and the optical cross-connect apparatus (OXC) for the optical routesetting apparatus 100 of the embodiment, it is possible to provide anoptical add-drop multiplexing apparatus (OADM), an optical cross-connectapparatus (OXC) and an optical communication network system, which areall excellent in reliability, availability and serviceability.

In the optical route setting apparatus 100 shown in FIG. 7, the alarmmask 303 is constructed in such a manner that, by using the alarmregister 352 and the mask register 353, the alarm issuing unit 351obtains a logical product of bits (“1” or “0”) stored in thecorresponding areas of both registers, and then issues an alarm.However, by using the CPU 342 to execute a program, alarm processingsetting an alarm mask can be executed. This processing is now describedby referring to FIGS. 14 to 17. In this case, as shown in FIG. 14, theoptical route setting apparatus 100 includes no constituent element ofalarm masks 303. In the alarm management memory 344, in addition to themasking period of each alarm of FIG. 9, programs as shown in FIGS. 15 to17 are prestored. The CPU 342 performs alarm processing and switching ofthe optical switch 300 by reading and executing the programs of thealarm management memory 344. The CPU 342 initializes the programs uponreading in step 151 of FIG. 15, inputs a loop, and executes alarmprocessing (step 152) unless there is a route switching request (step153) from the unillustrated supervisory and control system (OpS) orresetting (step 155). The alarm processing is finished without anychanges if the CPU 342 has received no alarms from the alarm interfacecircuit 353 (step 161) as shown in FIG. 16. If the CPU 342 has receivedan alarm, the CPU 342 itself determines a necessity of processing forrecovery from a trouble. If the processing for recovery is necessary,then the CPU 342 performs an operation for switching the optical switch300 to a predetermined route (steps 162 and 163). When necessary, forexample when a trouble cannot be solved by a switching operation in theself optical route setting apparatus 100, notification (steps 164 and165) is made to the not-shown supervisory and control system (OpS), andthe process is finished. In this case, the process proceeds to step 153of FIG. 15, a proper switching request is received from the not-shownsupervisory and control system (OpS), and a switching operation iscarried out (step 154).

In a case where a route switching request is received from the externalapparatus, the switching operation (step 154) or the switching operationfor recovery (step 163) is carried out in a manner shown in FIG. 17.First, the CPU 342 reads a masking period of each alarm from the alarmmanagement memory 344 for each output port, starts masking, and actuatesthe timer 346. In this case, a configuration can be made in such amanner that an area is previously provided in the alarm managementmemory 344 to write-in presetting of an alarm mask for each output port,and the CPU 342 sets a mask for each writing of “0” or “1” in this area.In a mask set state, a switching command is outputted to the opticalswitch 300 (step 173). Accordingly, the optical switch 300 executesroute switching. The CPU 342 proceeds to step 152, and executes a flowof alarm processing 152 shown in FIG. 16. However, if the CPU 342receives an alarm from the alarm interface circuit 353 in step 161,during the alarm masking period of the port in step 172, processing isexecuted determining that there are no alarms. The CPU 342 receives maskreleasing time information from the timer, and releases a specified maskwhen the mask releasing time is reached (steps 175 and 176). Forexample, the mask is released by writing “1” indicating mask releasingin the alarm mask writing area of the alarm management memory. Regardingthe alarm of the mask-released port, if the CPU 342 receives an alarmfrom the alarm interface circuit 353 in the alarm processing of step152, the process proceeds to step 162 in the flow of FIG. 16, andrecovery is made when necessary. When the masking time of a last alarmmask reaches a releasing time (step 175), alarm processing is executed(step 152), all the masks are released (step 174), and a result of theswitching is written in the switching information memory 343 (step 177).Thus, the switching operation of the optical routes is finished

As described above, according to the configuration of FIGS. 14 to 17,since the alarm mask can be realized by software, the optical routeswitching apparatus 100 of the embodiment can be provided with a simpleconfiguration of the apparatus. In addition, the programs of FIGS. 15 to17 are stored in the memory 344 of the control unit 305 of the existingoptical route switching apparatus, and the CPU 342 executes theseprograms. Thus, the existing optical route switching apparatus can beused to achieve the operation of the optical route switching apparatus100 of the embodiment.

According to the embodiment, there are four factors to be detected,i.e., the optical signal power failure, the error rate deterioration,the operation clock stepping-out, and the frame stepping-out. However,factors are not limited to these, and the number of factors can bereduced/increased as occasion demands. In any case, for each of thefactors, a proper masking period is preset.

Description has been made for the case where the optical route switchingapparatus 100 of the embodiment uses, as the optical switch 300, themechanical optical switch causing an optical power failure during theswitching operation. However, the optical route switching apparatus 100of the embodiment can use an optical switch causing no optical powerfailures during switching. For example, an optical switch based on anelectrooptical effect and a thermooptical effect can be used. In such anoptical switch, no optical power failures occur during a switchingoperation, but operation clock stepping-out and frame stepping-out occursimilarly to the embodiment. Thus, at least an operation clockstepping-out alarm and a frame stepping-out alarm are monitored as alarmfactors, and masking for predetermined periods is applied during theswitching operation of the optical switch, thus it is made possible toprevent the operation clock stepping-out and the frame stepping-out bythe normal switching operation of the optical switch from beingerroneously recognized as occurrences of troubles. Therefore, there isno possibility of starting a new route switching operation or the likefor recovery from a trouble despite the normal switching operation, thusmaking it possible to provide a highly reliable optical route settingapparatus. Moreover, a masking period is set properly for each alarm,then detection accuracy for an occurrence of a real trouble is notreduced, thus it is made possible to provide a highly reliable routeswitching apparatus.

As described above, according to the present invention, it is possibleto provide a highly reliable optical communication network system forperforming route switching of an optical signal without any changes ofthe optical signal, which is capable of preventing notification of anerroneous alarm during a switching operation of an optical switch.

1. An optical switching apparatus comprising: an optical switch forswitching and setting routes of an optical signal without beingconverted; a control unit for instructing said optical switch to executean operation of switching said routes; a performance monitor fordetecting performance of the optical signal having a route set by theoptical switch, and issuing an alarm; and an alarm masking unit forreceiving said alarm from said performance monitor, and passing saidalarm to said control unit, wherein said performance monitor issues saidalarm when said performance detected is deteriorated from predeterminedperformance, and said alarm masking unit masks said alarm issued fromsaid performance monitor for a predetermined masking period from astarting time of said operation of switching by said optical switch, andinterrupts an input of said alarm to said control unit.
 2. The opticalswitching apparatus according to claim 1, wherein said performancemonitor detects said performance of said optical signal for a pluralityof predetermined factors, and outputs the alarm for each of saidplurality of factors, said masking period of said alarm masking unit isdecided for each of said plurality of factors of said performancemonitor, and said alarm is masked for each of the plurality of factors.3. The optical switching apparatus according to claim 1, wherein saidplurality of factors include at least one of optical power, operationclock synchronization, frame clock synchronization, and an error rate ofsaid optical signal.
 4. The optical switching apparatus according toclaim 2, wherein said optical switch includes output routes amounting toN in number, said performance monitor is disposed for each of the numberN of output routes, said alarm is issued for said optical signal of eachof said number N of output routes regarding each of said plurality offactors, and said alarm masking unit masks said alarm for each of saidplurality of factors for each of said number N of output routes.
 5. Theoptical switching apparatus according to claim 4, wherein said alarmmasking unit includes a mask register, an alarm register and an alarmissuing unit, said mask register and said alarm register have bit areasrespectively corresponding to said plurality of factors for each of saidnumber N of output routes, in said bit area of said mask register, foreach of said factors of said corresponding output route, a bitindicating “MASKED STATE” is written during said masking period, and abit indicating “MASKING RELEASED” is written when said masking is in areleased state, in said bit area of said alarm register, for each ofsaid factors of said corresponding output route, a bit indicating “THEREIS ALARM” is written when said alarm is issued from said performancemonitor, and a bit indicating “NO ALARM” is written when no alarms areissued, and said alarm issuing unit issues said alarm to said controlunlit when said bits respectively indicating “MASK RELEASED” and “THEREIS ALARM” are written in said corresponding bit areas of said maskregister and said alarm register.
 6. An optical switch control apparatusfor instructing an optical switch for switching and setting routes of anoptical signal without being converted to execute a route switchingoperation, comprising: an alarm receiver for receiving an alarm issuedfrom an external performance monitor for detecting performance of saidoptical signal having said route set by said optical switch; and analarm masking unit for masking said alarm received by said alarmreceiver for a predetermined masking period from a starting time of saidswitching operation of said optical switch.
 7. The optical switchcontrol apparatus according to claim 6, wherein said externalperformance monitor detects said performance of said optical signal fora plurality of predetermined factors and issues said alarm for each ofsaid plurality of factors, said masking period of said alarm maskingunit is decided for each of said plurality of factors of said externalperformance monitor, and said alarm is masked for each of said pluralityof factors.
 8. An optical switching apparatus comprising: an opticalswitch for switching and setting routes of an optical signal withoutbeing converted; and an alarm masking unit for masking an alarm issuedfrom an external, performance monitor for detecting performance of saidoptical signal having said route set by said optical switch, whereinsaid alarm masking unit includes an alarm receiver for receiving saidalarm issued from said external performance monitor, and masks saidalarm received by said alarm receiver for a predetermined masking periodfrom a starting time of a switching operation of said optical switch. 9.The optical switching apparatus according to claim 8, wherein saidexternal performance monitor detects said performance of said opticalsignal for a plurality of predetermined factors and issues said alarmfor each of said plurality of factors, said masking period of said alarmmasking unit is decided for each of said plurality of factors of saidexternal performance monitor, and said alarm is masked for each of saidplurality of factors.
 10. An optical communication network systemcomprising: an optical switching apparatus, wherein said opticalswitching apparatus includes an optical switch for switching and settingroutes of an optical signal without being converted, a control unit forinstructing said optical switch to execute an operation of switchingsaid routes, a performance monitor for detecting performance of theoptical signal having said route set by the optical switch, and issuingan alarm, and an alarm masking unit for receiving said alarm from saidperformance monitor, and passing said alarm to said control unit; saidperformance monitor issues said alarm when said performance detected isdeteriorated from predetermined performance, and said alarm masking unitmasks said alarm issued from said performance monitor for apredetermined masking period from a starting time of said operation ofswitching by said optical switch and interrupts an input of said alarmto said control unit.
 11. The optical communication network systemaccording to claim 10, wherein said performance monitor detects saidperformance of said optical signal for a plurality of predeterminedfactors, and outputs said alarm for each of said plurality of factors,said masking period of said alarm masking unit is decided for each ofsaid plurality of factors of said performance monitor, and said alarm ismasked for each of said plurality of factors.
 12. An optical switchingapparatus comprising: a control unit; an optical switch for settingroutes of an optical signal; and a performance monitor for detectingperformance deterioration of said optical signal having be set a routeby said optical switch, and issuing an alarm, wherein said control unitincludes means for controlling route setting of said optical switch, andmeans for distinguishing the alarm between (1) one caused by a trouble,and (2) one caused by the route setting operation of said optical switchand controlling issuance of said alarm to the other connected apparatus.